can't figure out why part of UV map is stretching
Your topology is messy, when you unwrap Blender has no choice but to connect the faces, which gives stretched UVs. If we take a look at the corner we first see that the vertices are not connected, the face continues inwards:

Delete it, then you see that this edge floats:

Delete as well and cut again with the knife:

Now delete the face on the left and see, you have inner faces, delete them:

Now fill the hole:

Now unwrap again and your issue has disappeared:

There may be some other issues in your topology though, make sure that you don't have other inner faces, floating edges, etc... Another way to fix your UV problem quickly would have been to select one of the faces, then press ShiftG &gt; Select Similar Normal to select all the faces of this wall, switch to side view and U &gt; Project From View, but it would have left all the topology issues: